Magnetically actuated impact devices, especially those used as print hammers in high speed printers, are well known in the art. They utilize as their driving mechanism the deflective force experienced by a conductor when an electric current is passed through it while it is in the influence of an external magnetic field.
A conductive coil of particular cross-sectional configuration having n turns may be thought of as a series of n parallel conductive loops each of the same cross-section, aligned one with another in parallel planes along an axis which is normal to the planes in which the loops are disposed. Thus, for a coil of any cross-sectional configuration, the coil approximates a series of loops disposed in parallel planes. Thus, for example, a coil of rectangular cross-section having n turns can be visualized as a series of n parallel rectangular loops, each loop of which has two pairs of parallel legs. The plane in which the coil is wound for a coil of any cross-sectional configuration can be defined and used throughout this specification as the "coil plane". The axis about which the coil is wound is normal to the coil plane, and can be defined and used throughout this specification as the "coil axis".
Typically, conventional magnetically actuated impact devices attain a static magnetic field by means of stationary permanent magnets, or electro magnets, with opposite poles presented toward each other along an axis. These magnets are spaced apart from one another to provide a gap into which an impulse magnetic field generating means comprising a generally planar conductive coil is positioned for movement along a path parallel to the coil plane and for operative association with an impact member. The coil may be mounted directly on a one-piece impact member, or on one part of a multiple-piece impact member, so that displacement of the coil due to interaction of the static field with the impulse field when the coil is electrically excited is transferred directly into movement of the impact member toward a target. The coil may alternately be positioned to activate an interposer or split mechanism when electrically excited, so that movement of the impact member toward the target is effected indirectly by the coil. In the conventional actuation of impact devices, the static magnetic field component which interacts with the impulse magnetic field set up by the coil to cause coil displacement is the magnetic field component which is normal to the coil plane.
Development of magnetically actuated impact devices for use in the printer field is particularly troublesome because modern printing speeds of more than 2,000 lines per minute require, among other printing parameters, extremely rapid hammer travel time, low dwell time, and fast recovery time so that good print character resolution may be achieved.
Representative state-of-the-art magnetically actuated impact devices used as print hammers are disclosed in Helm, U.S. Pat. Nos. 3,172,352; Papadopoulous, 3,568,593; Lenders, et al, 3,735,698; and Wassermann, 4,014,258. These hammers are typically employed in side-by-side hammer arrays along a print line, wherein one coil is mounted on each hammer for selective activation by electrical current excitation of the coil. One print hammer may be provided for each print character position along the print line; or conventional space-sharing or time-sharing arrangements may be utilized so that one hammer services a plurality of print character positions.
Each of the above listed prior art references discloses a magnetically actuated impact device wherein a coil is supported on a movable impact member and positioned within a static magnetic field generated by means of paired permanent magnets. Passing an electric current through the coil induces an impulse magnetic field near the coil which interacts with the static magnetic field causing displacement of the coil and thereby the impact member toward the target. For each reference, the direction of travel of the coil is along a path parallel to the coil plane.
For a side-by-side print hammer array in the print hammer arrangements disclosed in Helms, Papadopoulous and Lenders et al, each coil is supported on its respective impact member with the coil axis parallel to the horizontal print line of the typed page. The coil plane in such configurations is perpendicular to the printed page with the active legs of the coil being those legs which are parallel to the page but perpendicular to the print line. Thus, by application of the right hand rule, the impact members are driven towards the page. In the side-by-side print hammer configuration of the type disclosed in the above identified Wassermann patent, the coil plane is also perpendicular to the printed page, but the coil axis is perpendicular to the horizontal print line of the page and the active coil legs are those which are parallel to the horizontal print line.
Because typical printer output is ten or more print characters per inch, magnetically actuated impact device design for side-by-side deployment in printers is subject to severe space limitations. The necessary packing densities in printers of the type disclosed in Helms are achieved by very thin coils fitted within narrow gaps between adjacent magnets. The number of turns in such coils is necessarily limited and clearance between the sides of the coils and the surfaces of the magnets is nominal. One common configuration employs wire which is flat in the coil plane as a means to conserve space. The very thin coils are relatively expensive to manufacture and considerable care and accuracy is required in assembly of such printers in achieving proper alignment of the coils within the narrow gaps. Further, the removal of the heat that builds up when the coils are repeatedly energized during high speed operation becomes a significant problem in closely packed designs. The geometry of these arrangements does not permit side stepping or column sharing of hammers unless the field generating means is moved as well.